One who steers or one entering into conspiracy (5)
I believe the answer is:
pilot
'one who steers' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one entering into conspiracy' is the wordplay.
'one' becomes 'I' (Roman numeral).
'entering into' indicates putting letters inside.
'conspiracy' becomes 'plot' (I've seen this before).
'i' placed into 'plot' is 'PILOT'.
'or' is the link.
Can you help me to learn more?
(Other definitions for pilot that I've seen before include "Steer (a ship or plane)" , "Driver of aircraft" , "He steers ship or plane" , "Person in charge of aircraft" , "air traffic controller?" .)